Semiconductor chips include contact pads that are electrically connected to external circuitry in order to function as part of an electronic system. The external circuitry is typically a lead array such as lead frame or a support substrate such as a printed circuit board. Electrical connection between the chip and the external circuitry is often achieved by wire bonding, tape automated bonding (TAB) or flip-chip bonding. For instance, with flip-chip bonding, ball grid array (BGA) packages contain an array of solder balls to mount on corresponding terminals on a printed circuit board, and land grid array (LGA) packages contain an array of metal pads that receive corresponding solder traces mounted on corresponding terminals on a printed circuit board.
Semiconductor chips include power semiconductor devices such as power diodes (such as PN diodes and Schottky diodes) and power transistors (such as MOSFETs, JFETs, IGBTs, BJTs and thyristors). For instance, power MOSFETs have the power handling capability of bipolar transistors and the advantages of an isolated gate. As a result, power MOSFETs have almost completely replaced bipolar transistor in power applications.
Power semiconductor devices, however, generate large amounts of heat. Therefore, power semiconductor devices are usually attached to a heat sink to dissipate the heat to the external environment.
Heats sinks are usually thermally connected to chips by a packaging interface. Furthermore, heat sinks may also protect the chip from the external environment (moisture, dust, etc.)
Heat sinks are available in a wide variety of structures and materials. For instance, a copper case is thermally connected and mechanically attached to a chip by a die attach epoxy, and the copper case provides a heat sink as well as a protective lid for the chip. The copper case is exposed, or alternatively, the copper case contacts and is sandwiched between the die attach epoxy and an aluminum heat sink with vertically extending comb-like fins. However, copper has a thermal conductivity of 393.6 watts per meter-° K, and aluminum has a thermal conductivity of 218 watts per meter-° K. Although these thermal conductivities are relatively high compared to ceramic materials such as aluminum oxide, they are not high enough to provide optimal heat dissipation.
Therefore, there is a need for improved heat sinks for chips and power semiconductor devices that have high performance, high reliability, low thickness and low manufacturing cost.